5 speed swap

[css1634]

Picked up my first datsun a 77 few months ago, it has a 4 speed, though it came with a 5. I found one today out of a 79 280 and through reading i belive it fits. Question is it worth changing back? Lower rpms, maybe slight mpg boost. Trans is 250 and good shape

[css1634]

Its a 620, noticed i left that out

[Guest Rick-rat]

My opinion, not that anyone cares, more work than it's worth, I changed to 3.89 rear  gears and I love it with my 4 speed. But I'm sure all the experts will say to change it out :thumbup:  Welcome to Ratsun

[datzenmike]

The '77 had a wide ratio 4 speed with a very low 3.592 first gear and no over drive.

 

The '79 280zx is a mid ratio with a 3.321 first and a 0.864 overdrive 5th.

 

This zx transmission is identical to the '77-'78 620 optional 5 speeds those years, so it will drive the same as a 5 speed 620 of those years. You will notice a very very slight increase in starting effort in first gear as it is taller. Once rolling the gears are slightly closer together with less RPM drop. Forth gear in both transmissions will be exactly the same and out on the highway the RPMs in 5th will be 13.6% lower. Remember 5th gear is not there to go faster, (likely it will go faster in 4th anyway) it's for reducing cruise speed RPMs. If carrying a load in the back, climbing a steep hill or passing, always down shift to a lower gear.

 

1/ To keep your speedometer the same, swap your current 4 speed speedometer pinion sleeve into the 4 speed. The pinion sleeve is always matched to the differential and these two must be kept together in the same vehicle.

 

2/ To keep your clutch and pressure plate working properly use your 4 speed clutch release bearing housing or sleeve on the 5 speed. Like the speedometer pinion the sleeve is matched to your pressure plate size and these two things must also stay together in the same vehicle 

 

 

Yes it will fit perfectly but post a picture of this 'mystery' 5 speed just to be sure.

[wayno]

Mike meant to say swap the 4 speed speedo gear/pinion sleeve into the 5 speed so your speedo reads the same as it does now.

 

The '77 had a wide ratio 4 speed with a very low 3.592 first gear and no over drive.

 

The '79 280zx is a mid ratio with a 3.321 first and a 0.864 overdrive 5th.

 

This zx transmission is identical to the '77-'78 620 optional 5 speeds those years, so it will drive the same as a 5 speed 620 of those years. You will notice a very very slight increase in starting effort in first gear as it is taller. Once rolling the gears are slightly closer together with less RPM drop. Forth gear in both transmissions will be exactly the same and out on the highway the RPMs in 5th will be 13.6% lower. Remember 5th gear is not there to go faster, (likely it will go faster in 4th anyway) it's for reducing cruise speed RPMs. If carrying a load in the back, climbing a steep hill or passing, always down shift to a lower gear.

 

To keep your speedometer the same, swap your current 4 speed speedometer pinion sleeve into the 4 speed. The pinion sleeve is always matched to the differential and these two must be kept together in the same vehicle.

[datzenmike]

Well the sleeve contains the plastic toothed gear and is easier to change as a unit than to remove the plastic gear... but I think we are on the same page here.

 

The '77 should have a RED 20 tooth gear.

 

The 280zx should have a 16, 17, 18 or 19 tooth gear depending on model.

[flatcat19]

No one asked which 4 speed he has.

 

Truck came with a 5 speed

But was swapped to a 4.

 

Measure the length of your trans.

If 26ish inches you will need a late 620 drive line.

 

If 31"-rock it.

[datzenmike]

Ha ha I thought the 5 speed came with the 4 speed truck. My bad.

 

Unless a LOT of work was done, the 4 speed should be the same length as the original 5 speed it replaced. Replace with confidence.

[Dolomite]

I changed to a zx 5 speed in my 510, better freeway mileage and much lower rpm at freeway speeds.

[css1634]

Thanks for the info, i plan to pull it next friday. So the pressure plate and disc will interchange as long as throw out bearing and pressure plate are matched?

[datzenmike]

Take out 4 speed.

 

Remove the release bearing and carrier that clips on the clutch lever.

 

Clip the release bearing from the 4 speed onto the 5 speed clutch arm.

 

Instal 5 speed.

 

 

 

(you really should replace with a new bearing)

[css1634]

Thanks for the reply, from reading it appears you are the goto man on trans questions. Thanks

[datzenmike]

Sometimes I get lucky with a guess.